The present invention relates to a doped fiber amplifier arrangement and, specifically, to a dual-stage amplifier with a passive pump splitting arrangement which allows low noise operation and variable signal gain with a single pump laser.
Erbium doped fiber amplifiers have revolutionized lightwave systems. These amplifiers have replaced detection and regeneration systems in long transglobal systems where fiber attenuation and dispersion act to degrade the transmitted signal. Further, fiber amplifiers are utilized in wide area signal distribution systems where distribution losses become appreciable.
In practice, the fiber amplifier is placed in-line with the signal carrying fiber to allow the optical signal to enter the Erbium fiber and directly experience gain. Connectors or fusion splices may be used to attach the amplifier to the signal fiber. Input and output isolators are typically utilized in unidirectional systems to control the direction of signals and amplified spontaneous emissions from the Erbium gain medium.
Topologically, fiber amplifiers take many forms. Multiple stages may be utilized to optimize performance. Additionally, either 980 nm or 1480 nm laser sources are typically utilized to co-, counter, or co- and counter-propagate pump power within each Erbium fiber amplifier stage. Wavelength multiplexers are utilized to couple the pump power to the Erbium fiber. Inter- and intra-stage isolators and filters maybe utilized to effect the noise and gain performance. In today's applications, a specific design consideration is flattening gain over the wavelength range of interest by selecting appropriate lengths of Erbium fiber or by utilizing an appropriate gain shaping filter.
Practical Erbium fiber amplifiers fall into one of three classifications: (1) post (or booster) amplifier, (2) in-line amplifier, or, (3) pre-amplifier. The post and pre-amplifier are often called terminal amplifiers. They are typically found at the ends (terminals) of a lightwave system, while the in-line amplifier is found mid-span in the system. Functionally, amplifier types differ in their input and output specifications.
The post amplifier operates in saturation to produce high optical output power (+17 to +24 dBm). The pre-amplifier is designed to deliver large small signal gain with minimal noise figure. Typically, a pre-amplifier will deliver &gt;30 dB of small signal gain with output power &gt;-10 dBm. The in-line amplifier delivers 10 to 20 dB of gain from input power levels between -20 and -10 dBm.
Erbium doped fiber amplifiers are commercially available from many suppliers. One or two stage amplifiers with one or more pumps per stage, either at 1480 nm or 980 nm are common. The bulk of commercial amplifiers have a limit range of variable gain with which to either stabilize output power or accommodate a range of input conditions.
An often used method of adjusting the gain state of a fiber amplifier is by dynamically adjusting the pump power in one or more stages. The most common way for controlling amplifier gain without sacrificing the total amplifier noise figure is by adjusting the pump power in the second stage of a dual-stage amplifier, i.e., leaving the first stage gain high enough to insure a reasonable total noise figure. This method usually allows approximately 22 dB of variable gain and requires a minimum of two pumps, one for each stage. Further, the second stage gain control is difficult due to the rapid nonlinear change of gain with decreasing pump power.
Therefore, a need remains in the art for an optical amplifier arrangement which combines the lower noise figure advantage of a dual-stage design with an extended range of variable gain and which is readily controllable with the cost advantages of a single pump laser.